1. Field of the Invention
The present invention relates to a piezoelectric element. More specifically, it relates to a piezoelectric element in which a component concentration in a thickness direction of a piezoelectric film is changed to obtain a pressure of a better displacement. Further, it relates to a process for producing the piezoelectric element. Still further, it relates to an ink jet recording head having the piezoelectric element.
2. Description of the Related Art
A piezoelectric film element having a piezoelectric film and an electrode is quite important as, for example, an actuator of an ink jet recording head, a micropump, a micromachine, a micromirror device or an ultrasonic vibrator. An ink jet recording head using a piezoelectric element has generally an ink chamber communicated with a nozzle and an ink supply unit, and it jets ink droplets from the nozzle by changing a volume of the ink chamber with a piezoelectric element. A piezoelectric element used in an ink jet recording head includes various types. For example, Japanese Examined Patent Publication No.53-12,138/1978 and Japanese Laid-Open Patent Publication No.6-40,030/1994 disclose a head using a piezoelectric element of a bending mode such as bimorph or unimorph. In the bending mode, a volume of an ink chamber is changed by bending a piezoelectric element constituting a part of a wall of an ink chamber like bimetal. In a part (substrate of a piezoelectric element) of a wall of an ink chamber, a lower electrode, a piezoelectric film and an upper electrode are laminated in this order. The piezoelectric film is polarized in a film thickness direction, and actuated in a d31 mode. For example, when an electric field of a direction in which the piezoelectric film is shrunk in a direction parallel to the surface is applied to both electrodes, the piezoelectric element is bent from the piezoelectric film toward the substrate in a perpendicular direction to the substrate. When an electric field of an opposite direction is applied, the piezoelectric film is extended, and the piezoelectric element is bend from the substrate toward the piezoelectric film.
In the piezoelectric element employed in this usage, satisfactory mechanical characteristics (pressure of a displacement) have to be obtained.
Japanese Laid-Open Patent Publication No. 10-290,035/1998 describes that a piezoelectric strain constant is stabilized by applying concentration gradients of zirconium and titanium in a film thickness direction and controlling heat generation of a piezoelectric film.
Japanese Laid-Open Patent Publication No.9-92,897/1997 describes a piezoelectric film in which a concentration of at least one of A and B of formula Pb(Axc3x97By)O3 (x+y=1) is changed in a film thickness direction of the piezoelectric film and a concentration of at least one of A and B shows a maximum value in an interface between the piezoelectric film and a lower electrode.
Japanese Laid-Open Patent Publication No. 10-139,594/1998 describes that in order not to decrease a piezoelectric strain constant, a piezoelectric film is composed of a large number of layers, a main layer being made of a three-component material comprising lead magnesium niobate, lead zirconate (PZ) and lead titanate (PT) and a partial layer being made of a two-component material comprising lead zirconate titanate (PZT).
The object of the present invention is to provide a piezoelectric element that provides a pressure of an excellent displacement as compared with an ordinary piezoelectric element. Further, the object of the present invention is to provide a process for producing the piezoelectric element. Still further, the object of the present invention is to provide an ink jet recording head having the piezoelectric element.
The present inventors have assiduously conducted investigations, and have consequently found that the aim of the present invention is attained by a piezoelectric film in which a maximum value of a concentration of A and/or B of a basic component Pb (A1/3 B2/3)O3 of a piezoelectric film is present in a region within 60% from an upper electrode side in a thickness direction of the piezoelectric film. This finding has led to the completion of the present invention.
That is, the present invention relates to a piezoelectric element comprising a substrate, a lower electrode formed on the substrate, a piezoelectric film containing Pb (Zr1-x Tix)O3 (0 less than x less than 1) and a valence compensation-type perovskite represented by general formula Pb(A1/3 B2/3)O3 (wherein A and B each represent a metal element capable of forming the valence compensation-type perovskite) as basic components formed on the lower electrode, and an upper electrode formed on the piezoelectric film,
wherein in any arbitrary regions in the thickness direction of the piezoelectric film, Pb (Zr1-x Tix)O3 and the valence compensation-type perovskite represented by the general formula Pb(A1/3 B2/3)O3 are contained,
in any arbitrary regions in the thickness direction of the piezoelectric film, a concentration of the valence compensation-type perovskite represented by the general formula Pb(A1/3 B2/3)O3 is within a range of 20 to 40 mole %, and
in the piezoelectric film, a concentration of A and/or B is changed in the thickness direction of the piezoelectric film and the maximum value of the concentration of A and/or B is shown in a region within 60% from the upper electrode side in the thickness direction of the piezoelectric film.
Preferable is a piezoelectric element in which in the piezoelectric film, the concentration of A and/or B is changed in the thickness direction of the piezoelectric film and the maximum value of the concentration of A and/or B is shown in a region within 40% from the upper electrode side in the thickness direction of the piezoelectric film.
By changing the concentration of A and/or B in the film thickness direction and providing the maximum value, the greater displacement of the piezoelectric film is obtained.
To obtain higher effects, Pb (Zr1-x Tix)O3 and the valence compensation-type perovskite represented by the general formula Pb(A1/3 B2/3)O3 are contained in any arbitrary regions in the thickness direction of the piezoelectric film, and a concentration of the valence compensation-type perovskite represented by the general formula Pb(A2/3 B2/3)O3 is within a range of 20 to 40 mole % in any arbitrary regions in the thickness direction of the piezoelectric film.
It is preferable that in the piezoelectric film, the concentration of A and/or B is changed in the thickness direction of the piezoelectric film and the minimum value of the concentration of A and/or B is shown in a region within 20% from the lower electrode side in the thickness direction of the piezoelectric film.
In the present invention, the thickness of the piezoelectric film is preferably between 1 and 25 xcexcm in view of the use as an actuator of an ink jet recording head. It is more preferably between 1 and 12 xcexcm. The piezoelectric film is composed of, preferably, 2 to 10 layers different in composition, more preferably 3 to 8 layers different in composition.
In the present invention, it is preferable that A represents an element selected from the group consisting of alkaline earth metals, Mn, Fe, Co, Ni, Cu and Zn and B represents an element selected from the group consisting of V, Nb and Ta. Examples of the alkaline earth metals include Mg, Ca, Sr and Ba.
Further, it is preferable that A represents an element selected from the group consisting of Mg, Ni and Zn and B represents an element selected from the group consisting of Nb and Ta.
It is preferable that the piezoelectric film of the piezoelectric element of the present invention further contains (Ba1-y Sry)TiO3 (0xe2x89xa6yxe2x89xa61) as a third component.
The piezoelectric film of the piezoelectric element of the present invention is preferably formed by a gaseous phase method in the point of view of durability of the elements. Examples of the gaseous phase method include a sputtering method, a vacuum deposition method, a CVD (Chemical Vapor Deposition) method and a laser ablation method. A RF magnetron sputtering method and a MOCVD (Metal Organic Chemical Vapor Deposition) method are more preferable. By forming the piezoelectric film by the gaseous method, the film having a more excellent mechanical strength is obtained and a durability of the piezoelectric element is improved.
The present invention also relates to a process for producing the piezoelectric element. A process for producing the piezoelectric element in the present invention comprises a step of forming a piezoelectric film on a lower electrode and a step of forming an upper electrode on the piezoelectric film,
the step of forming the piezoelectric film being conducted using a composition containing piezoelectric materials capable of forming the two basic components and, as required, the third component and/or precursors thereof.
Moreover, the present invention relates to an ink jet recording head having the piezoelectric element or the piezoelectric element produced by the process. The ink jet recording head comprises at least a nozzle, an ink chamber, an ink supply unit and a piezoelectric element, the nozzle and the ink supply unit being communicated with the ink chamber. More specifically it relates to an ink jet recording head using a piezoelectric element of a bending mode in which a substrate of the piezoelectric element serves also as a part of a wall of the ink chamber.
The ink jet recording head includes, for example, a head of a multi-nozzle in which plural nozzles are arranged in straight line or in zigzag fashion.
The ink jet recording head of the present invention can provide a satisfactory amount of displacement by using the piezoelectric element and can jet ink by variously changing a volume of one droplet upon controlling the amount of displacement. That is, an ink jet recording head is obtained which can control a volume of one droplet to be jetted over a wide range as compared with an ordinary head.
According to the present invention, a piezoelectric element is provided which gives a pressure of an excellent displacement. This piezoelectric element is preferably used in an ink jet recording head in particular, and this head can control the volume of one droplet to be jetted over a wide range as compared with an ordinary head.